<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
</head>
<body>
  <div id="clock"></div>
  <script>
    function send(){
      let xhr = new XMLHttpRequest()
      xhr.open('get', '/clock', true)
      xhr.onreadystatechange = function(){
        document.querySelector("#clock").innerHTML = xhr.responseText;
        // 在上一次的响应成功结束之后再发下一次请求
        send()
      }
      xhr.send()
    }
    send()
  </script>
</body>
</html>